The question of how the Universe began and how it’s going to end has puzzled mankind
for millennia. The ancient Greeks and many other civilisations believed that our Universe is eternal.
Aristotle was one of the first people to think differently and he suggested that
the Universe was confined in what he called a “heavenly sphere”. To everybody's surprise,
around 1930 it was discovered that the Universe is not infinitely old but has a finite age.
The astronomer Edward Hubble made the amazing observation that all galaxies move away from each other.
Therefore, one must assume that they had been closer together 1000 years ago than they are today, still
closer 1 million years ago and so on. Finally, at some time in the past, circa 13.7 billion years ago,
all matter in the Universe must have been compressed at an enormously large density and temperature.

The Big Bang. Image source: Counterbalance Foundation

Since the expansion from this initial state proceeds like a gigantic explosion, this theory
quickly became known as the Big Bang. Originally, the name Big Bang was used
ironically and coined by opponents of the Big Bang theory, because in the 1930’s it seemed
inconceivable to many people that the universe is not infinitely old. However, the name Big Bang is
used ever since because it appealed to the imaginations of scientists and laypersons alike.

At the moment, the Big Bang theory is the most accepted theory of the origin of the Universe -
but it is far from complete. The Big Bang theory explains how the Universe may have evolved after its
creation but there are many more cosmological questions. How did the Big Bang happen? And if our Universe
is expanding, what is it expanding into? Some of these questions are exciting new challenges for
astronomers, while others are just bewildering. For example, we know what happened to the Universe
from a fraction of a second after the Big Bang, but can’t fully explain the first fraction of a
second, where space and time were created. So if you asked what was before the Big Bang? Well nothing,
because space and time did not exist until the Big Bang, so there is no before the Big Bang
and there was no place for anything to be anyway.

The Big Bang theory states that all matter existed right from the start but it was just all in one place.
Before the Big Bang, there was no time and no space. Hubble discovered that the galaxies were moving
away from each other, which led to the idea of expanding space. This means that it is not the motion of the
galaxies themselves that drives them away from each other. It is space itself that is moving the galaxies apart.

In Albert Einstein’s General Theory of Relativity, he explains how every object in space is embedded
in space and it is space itself that is expanding, increasing the distance between the objects. All points
of matter and space were in one point before the Big Bang, which means
that the Big Bang happened everywhere.

It may be easier to think of a rubber balloon.
The balloon represents space (2-dimensional in this case). On the balloon you place hundreds of markers to
represent galaxies. If you blow up the balloon, the markers appear to move away from each other but
in reality, the balloon skin between each marker has stretched and moved all the 'galaxies' further apart.

Edward Hubble realised that all the galaxies were moving away from each other by observing light
from extremely distant sources. He found that this light had shifted towards the red end of the visible
spectrum. This can be explained by the Doppler Effect, which states that the light emitted
by a source moving away from the observer is observed with a lower frequency.

A lower frequency means that
the observed light looks more reddish than the light emitted by the source, and this effect is called
red shift.

Illustration of the Doppler effect. Image source: epicphysics.com

The Doppler Effect applies to all types of waves but it’s easiest to explain using sound.
Imagine a motorcycle moving towards an observer B.
Its engine is emitting sound waves (noise) in all directions but, as the
motorcycle travels forward, it’s catching up with the sound waves it released earlier, meaning that
the gap between each successive wave front gets smaller. This increases the frequency of the sound
waves which means B hears a higher pitched noise. Behind the motorbike, the effect is inverse, with the
spacing between wave fronts getting bigger as the motorcycle moves away from observer A, who hears a lower
pitched noise.

Light will experience a similar effect. For light-emitting objects moving towards the observer the waves
will be compressed together, meaning a higher frequency and lower wavelengths which appear blueish.
For galaxies moving away from us, the light will be shifted to lower frequencies (higher wavelengths) and appear redder.

This image shows that the light of far-away galaxies is red
instead of yellow because they are moving away from us.
Source: ESA/NASA

Theories of what the very early Universe was like are highly speculative as it is very difficult
to know in detail about things that happened 13.7 billion years ago.

We know that in the beginning
of the Universe the temperature was so high that all matter was dissolved into elementary particles
like quarks, electrons, and photons.
There were no atomic nuclei, atoms, solid states, planets or stars.
At this point in time only elementary particles could exist.

About a millisecond after the Big Bang, temperatures had cooled down enough to allow protons and
neutrons to form through the merging of three elementary quarks.

There are six different kinds of quarks. Two of them build up almost all matter: the up and down quark.
Three quarks can join to either form a proton (2 up + 1 down) or a neutron (1 up + 2 down).

In 1965 two American scientists, Penzias and Wilson, built a new radio antenna, of unprecedented
sensitivity, in order to measure satellite signals. They accidentally discovered a mysterious microwave
radiation coming from far out in space, but had no idea about its origin.
It was later found out that this radiation was created in the Big Bang. It is called cosmic
microwave background radiation and it raises the temperature of space from zero up to 2.7K.
The radiation is almost perfectly uniform in all directions.

You can even observe the background radiation yourself! Anyone who has have seen an analogue TV
before it has been properly set-up knows it will show 'static' on the screen.
About 10% of the flickering one sees is due to the background radiation. Of course this won’t
happen in the case of digital TVs.

Where does it come from?

After the creation of the first atomic nuclei of hydrogen, helium, and of traces of lithium,
these light elements remained embedded in a sea of electrons which originated from even earlier moments
in the history of the Universe. Each electron carries a negative electric charge whereas each proton
is positively charged. These protons and electrons are attracted to each other due to their opposite
charges and so the electrons try to form clouds around the protons.

However, the high temperature immediately dissolves the clouds and sends protons and electrons through
space on random paths. It took 300 000 years for the Universe to expand and cool down enough to reach
a temperature at which an electron cloud would remain stable. Nuclei with accompanying electrons are what we know
now as atoms.

The paths of photons (light particles) can be changed by collisions with electric charges but they do not
interact with neutral atoms. Therefore, the original Big Bang photons continued unperturbed on their paths once all
electrons had been incorporated in atoms, and we see those photons today as the cosmic background radiation.
Thus, this radiation gives a snapshot of the Universe at an "infant" age of 300 000 years.

The figure shows a detailed, all-sky picture of the infant universe created from nine years of
WMAP data.
The different colours in this picture show that the Universe 300 000 years after the Big Bang was not
the same everywhere. The colour differences indicate temperature fluctuations that correspond to the seeds that
grew to become the galaxies. Source: NASA/WMAP

Is there any proof of the Big Bang Theory ? Indeed, this theory is supported by the following
observations:

The expansion of the Universe by which galaxies move away from each other.
This means that all matter of the Universe must have been compressed at enormously
large temperature and density at the time of the Big Bang.

The abundance of light nuclei 2H, 3He, 4He and 7Li
can only be explained if one assumes that they were created in the first minutes after the Big Bang.

The observation of the cosmic background radiation coming from far out in space. The origin of this
radiation can be explained by assuming that it originated shortly after the Big Bang.

For about one billion years after the Big Bang, the Universe consisted only of hydrogen and helium in
gaseous form. There were no stars and no planets. Eventually, some the gas clouds began to collapse in on
themselves to form protostars which eventually became hot enough to ignite, creating the very first stars.
From stars, galaxies and planets the form. However, our Sun didn’t form until generations of stars had been
and gone, 8 to 9 billion years after the Big Bang.

A single grain of dust from the beginning of the Solar System Source: NASA

The surrounding dust and gas slowly merged together under the effect of gravitation, as larger pieces
attracted smaller pieces. As they grew, their weight eventually made them spherical and they became
planets.

We are fortunate that our Solar System formed quite late. Our planet is made out of rock which did not
exist in earlier times in the Universe. These heavy elements had to be made by other stars and then cast
out back into space when the stars died. This matter then collected around our Sun and formed not just the
Earth, but Mercury, Venus and Mars as well.

The solar system consisting of the Sun and the planets was formed from a cloud of gas and dust looking something
like this. Source: NASA

Even if one can measure how quickly the galaxies are moving away from each other, it is not easy to
predict what will happen in the future of the Universe. The expansion of the Universe is primarily powered by the Big Bang
but the matter in the Universe can gravitationally pull everything back towards the centre.

There are a number of possibilities and all depends on the density of the Universe:

If the density of the Universe is low, then the gravitational pull of the material in Universe
won’t be able to overcome the expansion so the Universe will continue to expand forever.
The lower the density, the quicker it can expand. This is known as the Open Universe model.

If the density of the Universe is high, then the Universe will expand for a while but slow down
as it goes until it reaches a maximum size. Then gravity will pull everything back in again until the
Universe collapses back into the singularity it started with in what we call The Big Crunch.
This is called a Closed Universe, as it has a finite size and can lead to another Big Bang
producing a new Universe.

If the density is exactly on the boundary between the two models, which we call critical density,
the Universe will still continue to expand but the rate of expansion will slow down.

The figure illustrates the three models of the expansion of the Universe.
Current observation seem to support the Open Universe model.

The Earth is about 4600 million years old. In the beginning, Earth was so hot that everything was molten.
After 500 million years Earth had cooled down enough so that oceans could form and the planet became habitable.
Then the first primitive forms of life appeared in the oceans. It then took an enormously long time
(about 4 000 million years) for life to develop from the first primitive life forms to plants,
then to animals, and finally to human beings.

Human beings have lived on Earth only for the last 4 million years. And only for less than 100 years mankind has an understanding of how
the Universe is structured up and how it came into existence in the Big Bang.

The conditions on Earth were just right for life. It is just the right temperature for liquid water,
close enough to the Sun to use its energy, oxygen for us to breathe and a moon to stabilise our climate.

This illustration represents the natural phenomena which have created life as we know it on Earth.
Source: California Space Institute

Many scientists think that there is life outside Earth. With billions of other star systems, it seems
absurd to believe that Earth is the only planet capable of harbouring life. In our solar system the most
probable locations where primitive life could exist are Mars and one of Jupiter’s moons.
On Mars, there is evidence that water was there in the past, and scientists think that on Europa,
a gigantic ocean lies below the icy surface.

As of 2011, well over 500 planets outside the Solar system have been discovered and more planets are being
found all the time. On some of those, there could be just the right conditions for life. Perhaps there could
be technical civilisations that are much further advanced than we are. However, some people feel that if there
were so many aliens out there, we would have seen some by now. This is called Fermi’s paradox,
because it was Enrico Fermi who first formulated this idea. One of the most plausible explanations for this
is that the huge distances between different advanced civilisations do not allow contact. Even if there were
1000 advanced civilisations out there, if we are all 1000 light-years apart it is unlikely that someone
will be coming to Earth for a visit.

Some scientists feel that it’s incredibly lucky that out of all the infinite possibilities of
how our Universe could have evolved, it just so happens, that it has evolved to create just the right conditions
for us to survive. For example, at the time of the Big Bang there was only a tiny surplus (one billionth) of
matter with respect to antimatter in the Universe. If the amounts had been exactly the same, all matter and
antimatter would have been annihilated and transformed into radiation, leading to a boring Universe without
anything solid, including ourselves. If the surplus of matter had been only a very tiny amount
larger or smaller than the actual value, no life would have been possible. If it had been a little larger,
the Universe would have collapsed due to gravitational attraction. There simply would not have been enough
time to develop life. On the other hand, if the surplus of matter had only been a little smaller the expansion
would have been so fast that no bound structures could have formed and the Universe would contain only
elementary particles.